Parthenogenesis in Sharks

What is Parthenogenesis?

The word parthenogenesis is derived from Greek and literally means “virgin birth.” An unfertilized egg will develop into a new individual –the new individual contains genetic information from its mother, and does not have a father. This phenomenon is observed in nature among some animals (insects, frogs, and sharks have been recorded in history).

Parthenogenesis was first described by Charles Bonnet, in the 18th century. By pricking frog eggs with a needle, Jacques Loeb was able to produce parthenogenetic frogs: some of the resulting embryos developed into completely healthy, adult frogs.

Parthenogenesis often results in a partially formed (or malformed) animal when attempted in mammals, though Gregory Pincus was able to induce parthenogenesis in rabbit eggs in 1936, using chemicals and temperature changes.

Understanding Ploidy

The terms Haploid and Diploid refer to the number of chromosome sets a species carries. Humans are diploid, as we have two of each chromosome. Some insects are haploid, such as male honeybees (drones). Haploid animals only have one copy of each chromosome. Gametes (egg and sperm cells) are typically haploid, with single chromosomes: this allows the sperm and egg cell to merge and form a diploid cell. Some plants and insects are tetraploid, which means they carry four copies of each chromosome.

Bee Colony Collapse

The Way Honeybees Reproduce

While parthenogenesis may sound like an odd or rare event in nature, it is actually the preferred form of reproduction for many species. Honeybees, for example, are able to sustain their population only through the ability of unfertilized eggs to develop. In honeybee colonies, the fertilized eggs become females, and the unfertilized eggs will develop into male drones. This is a process known as haploid parthenogenesis: the unfertilized egg has only half the number of chromosomes of a fertilized egg. The haploid bee will have the sex chromosomes XO, which causes the bee to become a male drone. Female bees have twice the number of chromosomes, with two X chromosomes to induce the development of female worker bees (or a Queen, if sufficient nutrition is provided to the larva).

Honeybee colonies that lack a male drone will eventually die out, as all of the larvae produced by the queen will be haploid and develop into drones. This is known as a drone brood, and the bee colony will degenerate and collapse without a sufficient supply of female worker bees.

Another way that drone broods form is when the colony lacks a breeding queen. The worker bees are unable to mate and will not typically produce young. In the absence of a fertile queen, however, the worker bees will begin producing eggs. These eggs are not fertilized, and will produce only male honeybees. These colonies are also doomed to collapse.

Types of Parthenogenesis

Type

Description

Observed In

Haploid

In haploid parthenogenesis, the unfertilized egg cell develops into an organism with half the number of chromsomes. This may result in a male (honeybee) or female (sheild bug).

Honeybees, rice, and wheat.

Diploid

In diploid parthenogenesis, an unfertilized egg combines with a polar body or another cell nucleus and develops into an organism with two copies of each chromosome. Diploid parthenogenesis is more common than haploid parthenogenesis.

Roundworms, fluke, and dandelions.

Exceptional (tychoparthenogenesis)

This term refers to an occurrence of parthenogenesis in a species that does not typically reproduce in this manner.

Sharks, frogs, mayflies

Normal or Physiologic

This term refers to parthenogenesis when it is the typical method of reproduction for an organism.

Honeybees, aphids, gall wasps, and many other insects.

Komodo Dragon Virgin Birth

A Komodo Dragon was born at the Chester Zoo in England, the result of a parthenogenetic birth. Komodo Dragons will have male offspring as the result of parthenogenesis. | Source

Komodo Dragon Virgin Births

Rare Occurrences in Nature

While parthenogenesis is common in insects, it is less common in fish and mammals. There have been documented cases of parthenogenesis in sharks, for example: Blacktip, Hammerhead, and White-Spotted Bamboo sharks have been reported to reproduce with this method.

The first documented case of a shark "virgin birth" was in Omaha, Nebraska in 2001. A female Hammerhead shark became pregnant, which was rather surprising since she had not been in contact with male sharks for over three years. The resultant offspring was confirmed to contain only the mother's DNA. A short time later, a Blacktip shark at a Virginia aquarium also became pregnant without the presence of males.

Both events resulted in a single pup from each mother - sharks typically deliver relatively large litters, so parthenogenesis is not a particularly good form of reproduction for sharks. In addition, all pups produced through this rare event will be female, as a Y chromosome is required from a fertilizing male shark to produce any male pups.

Komodo Dragons have also demonstrated the ability to reproduce using parthenogenesis. Unlike sharks which use an X and Y chromosome to determine gender, the reptiles have a ZW gender determination system. Female dragons are ZW and male dragons are ZZ. When a female Komodo Dragon's eggs develop parthenogenetically, the eggs are either ZZ or WW - the ZZ embryos develop into males, and the WW embryos fail to develop at all.

Due to this interesting ability, a female Komodo Dragon could create a breeding colony in isolation, as she would be able to lay a clutch of eggs - the developed male offspring could then mate with the mother and produce a colony of breeding dragons.

The use of parthenogenesis to breed Komodo Dragons is not advised, however, as the population would suffer from a condition known as a genetic bottleneck. When a breeding population lacks sufficient genetic diversity, it can become unstable as mutations increase through inbreeding.

Understanding Ploidy

Haploid organisms carry only one copy of each chromosome - this is the genetic profile of a honeybee drone. Humans and most other animals are diploid, and carry two copies of each chromosome. Parthenogenesis is possible for both conditions. | Source

Mammalian Parthenogenesis

Parthenogenesis was originally thought to be impossible for mammals, as the resulting offspring could never develop to term. Mammals are all diploid and require a sufficient amount of genetic information to develop into healthy, fully developed animals. Mammalian parthenogenesis has never been observed in the natural world, but scientists have managed to artificially induce it and to grow a healthy adult mouse, with the use of two female nuclei.

Inducing parthenogenesis in mammals requires the use of two cell nuclei, as all mammals are diploid and require two copies of each chromosome. Scientists at the Tokyo University of Agriculture in Japan fused two egg nuclei and managed to create a parthenogenetic mouse. The process is extremely difficult, however, as one of the egg nuclei had to be manipulated to contain the necessary genetic information for embryonic and fetal development. For example, a growth factor called IGF-2 is required for the development of the fetus, and the genetic information for this growth factor is provided in the sperm cell, not the egg cell. Mice were genetically modified to carry the genes for this growth factor in their egg cells, as the mouse embryos would have been unable to develop without it.

Parthenogenesis in Humans

Human eggs have the potential to become “activated,” or to begin division through parthenogenesis. An enzyme found in sperm, phospholipase-C-zeta (PLC-zeta), will induce the division of a human female's egg. There have been no scientifically documented cases of a human parthenogenetic egg cell developing into a fetus – these “activated eggs” simply develop to the blastocyst stage and become cysts or benign tumors. The blastocysts formed by the activated eggs look like very early embryos, and contain stem cells. As humans are diploid creatures, the use of the PLC-zeta enzyme would not ever allow for the development of a baby: the egg cell would remain haploid and only carry half the number of chromosomes required for normal development.

Parthenote Stem Cells

Uses of Parthenogenesis

Parthenogenetic human eggs might have a future for the growth of embryonic stem cells. No human egg cell has ever been able to develop into a fetus through parthenogenesis, but it is possible for these “activated eggs” to create new embryonic stem cell lines without the controversy endemic to embryonic stem cells gathered from early embryos. These stem cells are called parthenote stem cells.

Gynogenesis and Androgenesis

Some salamanders reproduce in a method that is similar to parthenogenesis. These salamanders, however, require the presence of sperm for the egg to activate. The sperm does not contribute any genetic material to the egg, but certain enzymes are required to trigger the egg to divide. This process is known as gynogenesis -all of the animals of a gynogenetic species are female, and must seek out a closely related species for mating to provide the necessary spermatic enzymes to activate the eggs.

The opposite of parthenogenesis is androgenesis, where an organism is able to fully develop from the male gamete. The resulting offspring are clones of their fathers - this phenomenon is observed in clams and other mollusks.

Questions & Answers

Questions must be on-topic, written with proper grammar usage, and understandable to a wide audience.

Question:

What drones are produced by both queen and worker bees?

Answer:

The worker bees do not produce any drones, as they do not have any progeny. When a queen bee lays an egg that is not fertilized, that egg will develop into a drone bee (XO), a haploid condition.

The genetic structure of a bee drone is fascinating. Hatched from an unfertilized egg, the bee drone has 16 chromosomes (a female honeybee has 32 chromosomes). Since the egg is unfertilized and genetic material from the queen is not contributed, each drone produces sperm that is identical in genetic structure to its own genome (the sperm is essentially a clone of the male's genetic material). This would cause a problem for the genetic diversity of the hive, but the queen bee resolves the issue by mating with anywhere from 10-20 drones during the course of 1-2 mating flights over a few days. The queen stores the sperm in an organ called a spermatheca, which allows the colony to have genetics from many different fathers.

There is one other way for a drone to develop, and it is rare. There is are 19 variants of sex-determining alleles, and two different varieties are required to produce a worker bee (female). If a fertilized egg happens to get the same allele from both the father and the queen bee, the resulting bee will develop as a drone. These are called "diploid drones" and the diploid drone is usually eaten by worker bees as soon as it emerges. The diploid drone cannot function to help the hive, and produces a "cannibalism" pheromone, which induces the other bees to cannibalize them.

Comments

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AUTHOR

Leah Lefler

8 months agofrom Western New York

The ability of some creatures to reproduce via parthenogenesis is absolutely fascinating, Julliett!

julliett

8 months ago

Good sort of knowledge about honeybees..

Kristen Howe

3 years agofrom Northeast Ohio

Leah, congrats on HOTD! This was a most fascinating and interesting hub on genetics for humans and animals. I never heard of these virgin births before. Thanks for sharing!

AUTHOR

Leah Lefler

5 years agofrom Western New York

In any case, the "offspring" created via scientific methods (like cloning) are generally not very robust and have multiple health problems. Of course, some animals naturally reproduce via parthenogenesis (honeybees, for example).

samowhamo

5 years ago

I agree. If nature hated men and did not men to exist than nature would never have created men (or any other male for that matter) in the first place nature does not make mistakes.

AUTHOR

Leah Lefler

5 years agofrom Western New York

What a horrible comment on the article, samowhamo - men and women are equally valuable to society. There are nutty people the world over.

samowhamo

5 years ago

There are some crazy women out there who would want to use parthenogenesis to eliminate men believing the world would be better without them. I was reading an article about whiptail lizards that reproduce by parthenogenesis and one of the commenters said this.

Once human females learn how to do this, they will no longer have a use for men. Thank Gawad. It's about bloody time.

Just because some species can do this that doesn't mean humans can or even should and even if men are not needed that doesn't take away their right to exist and they could still be wanted even if they are not needed. Here is a link to the article if you are interested.

I have never heard of Valerie Salonas, samowhamo - I'll have to look it up. There are more than a few crackpots out there - fortunately most of the world is filled with decent people. Parthenogenesis doesn't work with human eggs (to create viable embryos), so it isn't possible at any rate.

samowhamo

5 years ago

Well its not my kind of science fiction and besides its been done before in feminists fiction. I don't hate feminists (my best follower is a feminist) but I don't like the idea that some feminists have written about in fiction about single sex societies notably Valerie Salonas SCUM Manifesto which was her idea of how she wanted society without men which was not fiction (you should look it up if you don't know about it) but a I said I don't hate feminists (I do genuinely believe in gender equality) and my follower tells me that those particular feminists are only a minority.

AUTHOR

Leah Lefler

5 years agofrom Western New York

There is definitely a fair share of unbalanced people in the world! Anyone who blames one gender for the world's ills is bigoted. Of course, your concept would make for a great science fiction novel, samowhamo!

samowhamo

5 years ago

Thank you leahlefler. The reason why I asked is because there actually are some people out there (misogynist men and misandrist women) who would want to use things like this to get rid of men or women believing that the world would be a better place if men didn't exist or the world would be a better place if women didn't exist that's why it has ethical issues to it (I don't approve getting rid of men or women I believe every human being has the right to exist). In reality the world would not be any better if men or women didn't exist because you would still have problems and conflict.

AUTHOR

Leah Lefler

5 years agofrom Western New York

Parthenogenesis isn't really possible in humans, samowhamo, but I do agree that the ethics of any reproduction program should be examined before experiments are run. Still, I don't think the "regular" method of reproduction is ever going to be in jeopardy. Even though cloning is technically possible, for example, it would never take over the traditional method of reproduction due to the cost factor and health issues with the cloned offspring.

samowhamo

5 years ago

It is interesting but at the same time kind of scary to think that this could be applied to humans because it could label men or women redundant and no longer needed for reproduction so it does have ethical issues to it.

AUTHOR

Leah Lefler

6 years agofrom Western New York

Reptiles and fish sometimes reproduce in this manner, PaisleeGal, and insects often do. Mammals do not have spontaneous parthenogenesis, though it can sometimes be forced in laboratory conditions. It is a fascinating mode of reproduction!

Pat Materna

6 years agofrom Memphis, Tennessee, USA

Very good article!! So interesting!! Had no idea about mammals and fish being able to reproduce in this manner. Science has never been my strong point but I find it most interesting. Well Done! Voted up!

AUTHOR

Leah Lefler

6 years agofrom Western New York

I love science fiction, Sid - though science fact is often stranger than the wildest writer's imagination! Parthenogenesis is the preferred method of reproduction for many insect species, though it is not encountered in mammals (in nature). There are applications for the process in the laboratory, however, and it may provide a source of pluripotent stem cells that are free from some of the ethical concerns that plague embryonic stem cell research.

Sid Kemp

6 years agofrom Boca Raton, Florida (near Miami and Palm Beach)

Wow! Until now, I was aware of parthenogenesis in nature only in asexual microorganisms. And there are so many different genetic defintions of "male" and female. Fascinating! as Mr. Spock would say. (Dr. Spock would merely point out that there is no human parthenogenesis in nature.)

Edgar Rice Burroughs (of Tarzan fame) had a less-well-known series about an adventurer on Venus. He encountered a species of human who split down the middle to reproduce parthenogenetically. They were boring and uncreative, and his point was that sexuality and creativity are linked.

Another science fiction version had alien music trigger parthenogenesis. Only the music was accidentally broadcast across the galaxy, making women pregnant everywhere!

AUTHOR

Leah Lefler

6 years agofrom Western New York

Michael Crichton did have a medical degree, Teaches12345, and he surely learned about parthenogenesis in his undergrad courses. I think I heard about it in developmental biology for the first time - it is fascinating!

AUTHOR

Leah Lefler

6 years agofrom Western New York

It is really a strange idea, isn't it, Om? It is the method of choice among many insects and amphibians. A few reptiles do it, as do sharks. It is not heard of in mammals (in nature), though it is possible for laboratories to induce parthenogenesis in mammalian egg cells.

Dianna Mendez

6 years ago

Leah, this may just prove the movie Jurassic Park to have some valid facts within its storyline. I have heard about frogs and parthenogenesis, but not the others you mention here. Fascinating about the bees. Who knew? Thanks for the education here. Voted up, useful, interesting and awesome!

Om Paramapoonya

6 years ago

This was a really intriguing read, Leah. I had never heard about this type of reproduction in sharks or Komodo dragons. How fascinatingly bizarre!

AUTHOR

Leah Lefler

6 years agofrom Western New York

Thanks, carter06! Many insects use parthenogenesis to reproduce, but it is fairly rare in other animals!

Mary

6 years agofrom Cronulla NSW

This is amazing stuff...so interesting about the honey bee...my non biology/science brain is challenged:) but in a good way...great info & really well written...voted up & interesting...cheers

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